Methods and systems for detecting defects in serial link transceivers

ABSTRACT

Methods and systems for detecting defects in serial link transceivers. Defect detection includes detecting open circuits in one or more of the transmission lines, detecting short circuits between one or more of the transmission lines and a power supply, detecting short circuits between the transmission lines, or detecting short circuits across optional AC-couplings in the transmission lines. The detection can include direct or indirect detection of voltage or current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/760,583, filed Jan. 21, 2004, now U.S. Patent No. 7,460,585, whichclaims the benefit of U.S. Provisional Application No. 60/516,735, filedNov. 4, 2003, both of which are incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to serial link transceivers and, moreparticularly, to detecting defects in serial link transceivers.

2. Related Art

Serial link transceivers, including but not limited to differentialAC-coupled high-speed links, are susceptible to defects such asmanufacturing defects. Such defects can include shorts betweentransmission lines and ground, shorts between transmission lines andpower supplies, and open circuits in transmission lines.

There is thus a need for methods and systems for detecting defects inserial link transceivers.

SUMMARY OF THE INVENTION

The present invention is directed to methods and systems for detectingdefects in serial link transceivers. The invention can be implementedin, for example and without limitation, an AC-coupled differentialhigh-speed serial link transceiver (“transceiver”). In such atransceiver, a transmitter and receiver are coupled through differentialAC-coupled or DC-coupled transmission lines, and the receiver caninclude a common mode control circuit. In accordance with the invention,a monitoring system detects one of:

open circuits in one of the transmission lines;

short circuits between one or more of the transmission lines and a powersupply or ground plane;

short circuits between the transmission lines; and

short circuits across the AC-coupling capacitors.

Additional features and advantages of the invention will be set forth inthe description that follows. Yet further features and advantages willbe apparent to a person skilled in the art based on the description setforth herein or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by thestructure particularly pointed out in the written description and claimshereof as well as the appended drawings.

It is to be understood that both the foregoing summary and the followingdetailed description are exemplary and explanatory and are intended toprovide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The present invention will be described with reference to theaccompanying drawings, wherein like reference numbers indicate identicalor functionally similar elements. Also, the leftmost digit(s) of thereference numbers identify the drawings in which the associated elementsare first introduced.

FIG. 1 is a block diagram of an example differential serial-linktransceiver (“transceiver”) 100.

FIG. 2 is a block diagram of the transceiver 100, wherein a variety ofpotential defects are represented as shorts and/or open circuits.

FIG. 3 is a block diagram of the differential receiver 104, including acommon-mode control circuit Vcm 302.

FIG. 4 is a block diagram of the receiver 104, including an examplemonitoring system 402.

FIG. 5 is a flowchart of a method for detecting defects in a serial linktransceiver.

FIG. 6 is a flowchart of step 504 from FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

Introduction

FIG. 1 is a block diagram of an example differential serial-linktransceiver (“transceiver”) 100. The transceiver 100 includes adifferential transmitter 102, a differential receiver 104, and adifferential link 106. The differential link 106 includes differentialtransmission line 108 and 110. The differential transmitter 102 outputsVp on the one leg of the differential transmission line 108, and Vn onthe other leg of the differential transmission line 110. In the exampleof FIG. 1, the transmitter 102 and receiver 104 are AC coupled throughcapacitors 112 and 114, respectively. Alternatively, the capacitors 112and 114 are omitted and the transmitter 102 and receiver 104 are DCcoupled.

Defect Detection

The differential transceiver 100, or portions thereof, are typicallyimplemented on one or more printed circuit boards (“PCBs”). Defects inthe PCBs can be encountered during manufacturing and/or subsequenthandling. Defects in the PCBs can reduce the performance of thetransceiver 100 and/or render the differential transceiver 100inoperable.

FIG. 2 is a block diagram of the transceiver 100, wherein a variety ofpotential defects are represented as shorts and/or open circuits. Thepotential defects are described below.

The present invention is directed to methods and systems for detectingone of the defects represented in FIG. 2. Systems for detecting one ofthe defects represented in FIG. 2 include a monitoring circuit such as,without limitation, a voltage and/or current monitoring circuit.

The monitoring circuit can be implemented to directly or indirectlydetect a defect. A direct monitoring circuit can be coupled to either ofthe differential transmission lines 108 and 110. An indirect monitoringcircuit can be coupled to a circuit that supports the transceiver 100.For example, and without limitation, an indirect monitoring circuit canbe coupled to a common mode control circuit that supports thetransceiver 100. An indirect monitoring circuit has the advantage of notadding additional high speed circuitry to the transmission lines, andthus does not degrade system performance.

FIG. 3 is a block diagram of a differential receiver 104, including anexample common-mode control circuit Vcm 302. The common-mode controlcircuit maintains signals on the transmission lines 108 and 110 centeredabout a common mode voltage. Common-mode control circuits are well knownin the art.

Under normal operating conditions, the common-mode control circuit Vcm302 provides substantially zero DC current and zero AC current to thedifferential transmission lines 108 and 110. When there is a defect inthe differential transceiver 100, however, the common-mode controlcircuit Vcm 302 will provide a DC current and/or an AC current totransmission line 108 and/or to transmission line 110 to maintain signallevels thereon at desired common mode levels.

Referring back to FIG. 2, example potential defects and correspondingreactions of the common-mode control circuit Vcm 302 are now described.The invention is not, however, limited to transceivers that includecommon-mode control circuits. The invention is also not limited toAC-coupled transceivers.

The example potential defects include fourteen potential defects in thepositive and negative signal lanes (i.e., transmission lines 108 and110). The potential defects also include two potential shorts betweenthe positive and negative signal lanes, and two potential cross shortsacross the AC coupling capacitances 112 and 114. The example potentialdefects are now described in detail with reference to FIG. 2.

R1 p represents a potential open circuit in the transmission line 108between the transmitter 102 and the capacitor 112. When this occurs, thecommon-mode control circuit Vcm 302 provides an AC current that tracksVin in an attempt to maintain signals on the transmission lines at acommon mode voltage.

R2 p represents a potential short between the transmission line 108 anda power supply VDD between the transmitter 102 and the capacitor 112.When this occurs, the common-mode control circuit Vcm 302 provides an ACcurrent that tracks Vp.

R3 p represents a potential short between the transmission line 108 anda power supply VSS between the transmitter 102 and the capacitor 112.When this occurs, the common-mode control circuit Vcm 302 provides an ACcurrent that tracks Vp.

R4 p represents a potential short across the capacitor 112. When thisoccurs, the common-mode control circuit Vcm 302 sinks or sources a DCcurrent.

R5 p represents a potential short between the transmission line 108 andthe power supply VDD between the capacitor 112 and the receiver 104.When this occurs, the common-mode control circuit Vcm 302 provides an ACcurrent that tracks Vp.

R6 p represents a potential short between the transmission line 108 andthe power supply VSS between the capacitor 112 and the receiver 104.When this occurs, the common-mode control circuit Vcm 302 provides an ACcurrent that tracks Vp.

R7 p represents a potential open in the transmission line 108 betweenthe capacitor 112 and the receiver 104. When this occurs, thecommon-mode control circuit Vcm 302 provides an AC current that tracksVp.

R1 n represents a potential open in the transmission line 110 betweenthe transmitter 102 and the capacitor 114. When this occurs, thecommon-mode control circuit Vcm 302 provides an AC current that tracksVn.

R2 n represents a potential short between the transmission line 110 andthe power supply VDD between the transmitter 102 and the capacitor 114.When this occurs, the common-mode control circuit Vcm 302 provides an ACcurrent that tracks Vn.

R3 n represents a potential short between the transmission line 110 andthe power supply VSS between the transmitter 102 and the capacitor 114.When this occurs, the common-mode control circuit Vcm 302 provides an ACcurrent that tracks Vn.

R4 n represents a potential short across the capacitor 114. When thisoccurs, the common-mode control circuit Vcm 302 sinks or sources a DCcurrent.

R5 n represents a potential short between the transmission line 110 andthe power supply VDD between the capacitor 114 and the receiver 104.When this occurs, the common-mode control circuit Vcm 302 provides an ACcurrent that tracks Vn.

R6 n represents a potential short between the transmission line 110 andthe power supply VSS between the capacitor 114 and the receiver 104.When this occurs, the common-mode control circuit Vcm 302 provides an ACcurrent that tracks Vn.

R7 n represents a potential open circuit in the transmission line 110,between the capacitor 114 and the receiver 104. When this occurs, thecommon-mode control circuit Vcm 302 provides an AC current that tracksVn.

R1 d and R2 d represent potential shorts between the transmissions line108 and 110. When R1 d or R2 d occurs, no signal is presented at thereceiver 104.

R1 x and R2 x represent potential shorts or cross-faults across the ACcoupling capacitances 112 and 114. When R1 x occurs, the common-modecontrol circuit Vcm 302 provides an AC current that tracks Vn. When R2 xoccurs, the common-mode control circuit Vcm 302 provides an AC currentthat tracks Vp.

Defects in the differential transceiver 100 can reduce the performanceof the differential transceiver 100 and/or render it inoperable.Therefore, it is useful to know when there is a defect in thedifferential transceiver 100.

In many situations, it is sufficient to know that there is a defect inthe differential transceiver 100. It is often not necessary to know theprecise location of the defect, the precise nature of the defect, or theprecise number of defects. For example, IEEE 1149.1 is an IEEE supportedstandard directed to detecting PCB manufacturability issues usingon-chip monitoring functions. IEEE 1149.6 is an extension to the IEEE1149.1 standard directed to high-speed differential signal lines usingAC-coupling, such as in serializer/deserializer (“SERDES”) transceivers.IEEE 1149.6 stipulates that defects be detectable using a pass/failcriteria. IEEE 1149 is incorporated herein by reference in its entirety.

Accordingly, the present invention is directed to methods and systemsfor detecting the presence of at least one serial-link interconnectdefect.

Recall from above that the invention can be implemented by monitoringvoltage or current, directly or indirectly. An example is provided belowfor indirectly monitoring current. The invention is not, however,limited to this example. Based on the description herein, one skilled inthe relevant art(s) will understand how to detect a defect by monitoringvoltage or current, directly or indirectly.

In the example of FIG. 3, the transceiver 100 includes a common-modecontrol circuit Vcm 302. In accordance with the invention, currentprovided by the common-mode control circuit Vcm 302 is monitored toindirectly detect one or more of the conditions discussed above. Forexample, AC and/or DC current provided by the common-mode controlcircuit Vcm 302 is monitored. When the AC and/or DC current provided bythe common-mode control circuit Vcm 302 exceeds corresponding AC and/orDC current thresholds, a defect is declared. For AC and/or DC currentmonitoring, the corresponding threshold can be a fixed level.Alternatively, or additionally, the AC threshold is dynamicallydetermined relative to Vp and/or Vn, and/or relative to one or moreother variables.

Based on the description herein, one skilled in the relevant art(s) willunderstand that any conventional and/or yet to be developed currentand/or voltage monitoring methods and/or systems, and/or combinationsthereof, can be employed.

FIG. 4 is a block diagram of the receiver 104, including an examplemonitoring system 402. In the example of FIG. 4, the monitoring system402 is configured to monitor AC and/or DC current and/or voltage that isprovided by the common-mode control circuit Vcm 302. Current and voltagemonitoring systems are well known in the art.

In the example of FIG. 4, the monitoring system 402 is an indirectmonitoring system. In alternative embodiments, the monitoring system 402is coupled directly to one of the differential transmission lines 108 or110 to monitor current or voltage directly.

The monitoring system 402 includes one or more of a variety ofconventional and/or yet to be developed current monitoring systems,methods, and/or combinations thereof.

The monitoring system 402 includes one or more outputs 404 foroutputting results of the monitoring. The monitoring system 402 outputsa fault indication on the output(s) 404 when the monitoring system 402senses one or one or more current conditions indicative of the potentialdefects described herein.

In the example of FIG. 4, the monitoring system 402 is coupled betweenthe common-mode control circuit Vcm 302 and a common-mode controlcircuit resistance Rcm 304. Alternatively, the monitoring system 402 iscoupled between the common-mode control circuit resistance Rcm 304 andthe transmission lines 108 and 110. Alternatively, the monitoring system402 is coupled to a node between the common-mode control circuit Vcm 302and the common-mode control circuit resistance Rcm 304, or to a nodebetween the common-mode control circuit resistance Rcm 304 and thetransmission lines 108 and 110, so that the monitoring system 402 is notdirectly in the Vcm signal path. Thus, when the monitoring system 402 isimplemented as an indirect monitoring system, the invention does notrequire additional high-speed circuitry in the transmission lines 108and/or 110, and thus does not degrade dynamic performance of thedifferential transceiver 100.

Alternatively, the monitoring system 402 is coupled directly to thetransmission line 108 and/or 110.

FIG. 5 is a flowchart of a method 500 for indirectly detecting defectsin a serial link transceiver having a common-mode control circuit, asillustrated in FIGS. 3 and 4. The method 500 is described in terms ofmonitoring current from a common-mode control circuit Vcm. The inventionis not, however, limited to indirectly detecting defects and/or tocurrent monitoring. Based on the disclosure herein, one skilled in therelevant art(s) will understand how to implement the invention todirectly detects, and how to implement the invention to detect defectsbased on voltage monitoring, with and without a common-mode controlcircuit.

The method illustrated in the flowchart of FIG. 5 can be implementedwith circuitry described above, and/or with any other suitablecircuitry. The process begins at step 502, which includes sensing anoutput of the common-mode control circuit Vcm, without adverselyaffecting data transmission (e.g., without degrading dynamic performanceof the transceiver).

Step 504 includes outputting an indication of a defect when the sensedoutput of the common-mode control circuit Vcm exceeds a threshold. Step504 can be implemented to detect one or more of the potential defectsdescribed above. For example, FIG. 6 illustrates step 504 implementedwith steps 602-608.

Step 602 includes outputting an indication of a defect when AC output ofthe common-mode control circuit Vcm tracks Vin. Step 602 is useful, forexample, for detecting open circuits in the transmission line 108 andshorts between the transmission line 108 and a power supply. Step 602 isuseful in other situations as well, as will be apparent to one skilledin the relevant art(s) after reading the description herein.

Step 604 includes outputting an indication of a defect when AC outputfrom the common-mode control circuit Vcm tracks Vip. Step 604 is usefulfor detecting open circuits in the inverted transmission line 110 andshorts between the inverted transmission line 110 and a power supply.

Step 606 includes outputting an indication of a defect when no signal ispresented at the receiver and a current and/or voltage output isprovided by common-mode control circuit Vcm. Step 606 is useful fordetecting short circuits between the transmission lines.

Step 608 includes outputting an indication of a defect when a DC outputis provided by common-mode control circuit Vcm. Step 608 is useful fordetecting short circuits across AC couplings.

Conclusion

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed. Any such alternate boundaries are thus within the scope andspirit of the claimed invention. One skilled in the art will recognizethat these functional building blocks can be implemented by discretecomponents, application specific integrated circuits, processorsexecuting appropriate software and the like and combinations thereof.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

1. A differential receiver, comprising: a differential input configuredto receive a differential signal having an input common-mode voltage; acommon-mode control circuit configured to adjust the input common-modevoltage to a desired common-mode voltage by adding a correction currentto a portion of the differential input; and a monitoring systemconfigured to monitor the correction current provided to thedifferential input, and determine when the correction current exceeds athreshold, thereby representing a defect in a differential transmissionline coupled to said differential input.
 2. The differential receiver ofclaim 1, wherein the correction current is one of an AC correctioncurrent or a DC correction current.
 3. The differential receiver ofclaim 1, wherein the monitoring circuit outputs an indication of adefect in the differential transmission line when the correction currentexceeds the threshold.
 4. The differential receiver of claim 3, whereinthe indication identifies whether the correction current is an ACcorrection current or a DC correction current.
 5. The differentialreceiver of claim 3, wherein the indication identifies when the ACcorrection circuit tracks the changes in the differential signal.
 6. Theapparatus according to claim 3, wherein the monitoring circuit isconfigured to output an indication of a defect when a short circuitexists across an AC coupling in an AC-coupled differential transmissionlines.
 7. The differential receiver according to claim 1, wherein thedifferential transmission line includes AC-coupled differentialtransmission lines, and wherein the monitoring system detects shortcircuits across AC-coupling capacitors in one of the AC-coupleddifferential transmission lines.
 8. The differential receiver accordingto claim 1, wherein the monitoring system is coupled directly to one ofthe differential inputs.
 9. The differential receiver according to claim1, wherein the monitoring system is coupled indirectly to one of thedifferential inputs.
 10. The differential receiver according to claim 1,wherein the monitoring system is configured to output an indication of adefect upon an of the following conditions: AC correction current isdetected exceeding a predetermined threshold; DC correction current issensed exceeding a predetermined threshold; or no signal is received bythe differential receiver and a current is sensed by the currentmonitoring system exceeding a predetermined threshold.
 11. The apparatusaccording to claim 1, wherein the monitoring circuit is configured tooutput an indication of a defect when an open circuit exists in thedifferential transmission line.
 12. The apparatus according to claim 1,wherein the monitoring circuit is configured to output an indication ofa defect when a short circuit exists between the differentialtransmission line and a power supply.
 13. The apparatus according toclaim 1, wherein the monitoring circuit is configured to output anindication of a defect when a short circuit exist in the differentialtransmission line.
 14. In a differential receiver having a differentialinput, a method of detecting defects in a differential transmission linecoupled to the differential input, comprising the steps of: receiving adifferential signal having an input common-mode voltage at thedifferential input; adding a correction current to the differentialsignal to adjust the input common-mode voltage to a desired common-modevoltage; monitoring the correction current that is added to thedifferential signal, to determine when the correction current exceeds athreshold, thereby representing a defect in the differentialtransmission line coupled to the differential input.
 15. The method ofclaim 14, further comprising the step of outputting an indication of thedefect in the transmission line when the correction signal exceeds thethreshold.
 16. The method of claim 15, wherein the indication identifieswhether the correction current is an AC correction signal or a DCcorrection signal.
 17. The method of claim 16, wherein the indicationfurther indicates whether the AC correction current substantiallyfollows the differential signal.
 18. A differential receiver,comprising: a differential input configured to receive a differentialsignal having an input common-mode voltage; means for adjusting theinput common-mode voltage to a desired common-mode voltage by utilizinga correction signal; and means for monitoring the correction signal todetermine when the correction signal exceeds a threshold, therebyrepresenting a defect in a differential transmission line coupled tosaid differential input.
 19. The differential receiver of claim 18,wherein said means for monitoring includes means for indicating the typeof defect in the differential transmission line.
 20. The differentialreceiver of claim 19, wherein the means for indicating includes meansfor indicating whether the correction signal is an AC correction signalor a DC correction signal.